The present invention relates to a gas turbine engine, and particularly to a bypass turbofan engine nacelle assembly, and more particularly to a bypass turbofan engine nacelle assembly having a ported variable area fan nozzle (VAFN) with an acoustic system to reduce the total effective perceived noise level (EPNL).
Gas turbine engines which have an engine cycle modulated with a VAFN typically provide a smaller fan exit nozzle area during cruise conditions and a larger fan exit nozzle area during take-off and landing conditions.
The ported VAFN system may generate significant noise as upstream turbulence interacts with the leading edge of the VAFN. The upstream turbulence may result from turbulent boundary layers which expand from the upstream fixed nacelle wall, turbulence which evolves from the upstream fan exit guide vane (FEGV) wakes or endwall effects, and flow separation that may occur from the contour of the upstream nacelle wall. The physical mechanism for leading edge VAFN noise, which exhibits acoustic dipole behavior, is fundamentally different from traditional jet exhaust mixing noise, which exhibits acoustic quadrupole behavior. Additionally, this excess noise is not significantly reduced in forward flight as typical jet exhaust mixing noise. Thus, VAFN noise contributes toward the total effective perceived noise level. Accordingly, it is desirable to reduce VAFN noise.